Yeah, let's turn a single O(n) scan with a hash (the right way to do it) into an O(n log n) sort followed by another O(n) scan. Aside from the added inefficiency you've lost the original ordering if that needed to be preserved.

Update: Ah, a followup did note that the list was to be sorted. Still makes more sense to do the O(n) cull of duplicates first and then the O(n log n) sort of the smaller list.